Introduced support for external services registration in the ServiceProtocol

runtime/observatory/lib/src/elements/helpers/nav_menu.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:html';
 
-navMenu(String label, {String link: '', Iterable<Element> content: const []}) {
+navMenu(String label, {String link, Iterable<Element> content: const []}) {
   assert(label != null);
   assert(content != null);
-  assert(link != null);
   return new LIElement()
     ..classes = ['nav-menu']
     ..children = [
       new SpanElement()
         ..classes = ['nav-menu_label']
         ..children = [
           new AnchorElement(href: link)..text = label,
           new UListElement()..children = content
         ]
     ];
 }